The per-device price line in the buy-devices modal showed a
strikethrough 'Бесплатно' for users with a promo-group discount,
because original_price_per_device_kopeks was missing in the API
response and formatPrice(0) renders 'Бесплатно'. Now the
strikethrough is rendered only when original_price_per_device_kopeks
is present and non-zero, mirroring the existing guard on the total
price block.